The present invention relates to a method for detecting a lean limit by means of ionic current in an internal combustion engine mainly of an automobile driven in a lean burn zone in which the air-fuel ratio is high.
In recent years the necessity has been realized of driving an automobile with the air-fuel ratio of the mixture gas set at the lean side of the stoichiometric air-fuel ratio in order to improve fuel consumption. In internal combustion engines, an air-fuel ratio control device, such as described in Japanese Pat. Laid-open No. 62-162742, is well known. The air-fuel ratio control device first detects an engine load. If the engine is in a predetermined transient state, feedback control is conducted in accordance with the stoichiometric air-fuel ratio. If the engine is in a normal state, the amount of fuel supply is controlled in accordance with the air-fuel ratio set at the lean side of the stoichiometric air-fuel ratio. At the upstream side of a three-way catalyst converter in the exhaust system of the engine there is provided an air-fuel ratio sensor. The air-fuel ratio set at the lean side of the stoichiometric air-fuel ratio is controlled so as to approach a target air-fuel ratio in accordance with the output from the air-fuel ratio sensor.
It is known that torque fluctuation occurs in an engine as the air-fuel ratio is increased (see FIG. 10). In order to avoid torque fluctuation, the air-fuel ratio should be set below a certain value in the lean burn zone. Torque characteristics, as shown in FIG. 11, are peculiar to an engine or the environment in which the engine is driven, and the upper limit of the air-fuel ratio in the lean burn zone fluctuates with the driving environment or the engine. Accordingly, the upper limit of the air-fuel ratio in the lean burn zone should be adjusted for each engine or a particular driving environment, and the target air-fuel ratio is set with a certain safety factor taken into consideration.
The air-fuel ratio set with the safety factor as mentioned above poses a problem that fuel consumption is aggravated or NO.sub.x is increased. This problem may be solved by detecting the limit of torque fluctuation, and then delimiting the lean burn zone with a target air-fuel ratio set at a value lower than the air-fuel ratio at which the torque fluctuates. However, there is no effective way of detecting torque fluctuation, with resulting difficulty in controlling the engine near the upper limit of the lean burn zone.
The present invention is intended to solve these problems.